The present invention relates to bulk acoustic wave resonators. More particularly, the present invention relates to multi-resonator bulk acoustic wave filters with acoustic mirrors.
A thin film bulk acoustic wave (BAW) resonator is based on a layer of piezoelectric material, such as ZnO or AlN, and in some cases includes an acoustic mirror. Such a device converts sound waves to electric signals, and vice versa, and can be used as a filter in electronic circuits because of its frequency dependent electrical impedance. Typically, the acoustic mirror is formed from a combination of layers of materials of differing acoustic impedance. The acoustic mirror is built up on a substrate of for example glass by depositing its various layers of different materials so as to form a stack of layers of different materials on the substrate. Next, a bottom electrode is deposited on the acoustic mirror, and the piezoelectric material is then deposited on the bottom electrode forming a so called piezolayer. Finally, a top electrode is deposited on the piezolayer. The combination of top and bottom electrodes and the piezolayer forms what is called the resonator section of the device. The acoustic mirror serves to reflect acoustic waves created by the piezolayer in response to a voltage applied across the electrodes, thereby isolating the substrate from the piezolayer.
An example of a resonator including an acoustic mirror is disclosed in the article entitled xe2x80x9cDevelopment of Miniature Filters for Wireless Applicationsxe2x80x9d , IEEE Transactions on Microwave Theory and Techniques, Vol. 43, No. 12, December 1995. The acoustic mirror in such a resonator may include a lower layer having a low acoustic impedance and a thickness of approximately one-quarter wavelength, and an upper layer having a high acoustic impedance. In such a device, a layer pair serves as an xe2x80x9cimpedance transformer,xe2x80x9d since it can transform the acoustic impedance of a substrate to a very low value. In a device where each of the layers has a thickness of approximately one-quarter wavelength, the conversion factor of the pair of layers is equal to the square of the ratio of their respective impedances.
Besides BAW resonators including acoustic mirrors, it is known in the art to provide BAW resonators constructed on a membrane, with an airgap separating the resonator section from the substrate. The disadvantages of the membrane type approach are that it is difficult to produce the layers on top of the membrane so that they have sufficiently small mechanical stress, which would break or bend the membrane. In addition, the membrane structure is not very rugged mechanically, which complicates the handling and dicing of ready wafers (glass or silicon wafers, 4xe2x80x2 to 8xe2x80x2 in diameter, that are fully processed, containing thousands of resonator-based filters). Depending on the type of membrane, there may be limitations on the possible substrate material that can be used.
The acoustic mirror type of BAW resonator is clearly more rugged, since the entire structure is solidly mounted on the substrate. The mirror operates basically as a xcex/4 transformer, i.e. it consists of multiple pairs of alternating layers of high and low acoustic impedance materials, each approximately acoustically one quarter wavelength thick. Thus, the entire stack transforms the acoustic impedance of the substrate to a very low impedance at the mirror/bottom electrode interface, creating an acoustically reflective interface similar to the air interface in membrane type structures. The optimal operation of the mirror requires that the difference in the high and low impedance be as large as possible. The difference in the acoustic impedance of currently available dielectric films is not large so that a large number of layers must be used for an all-dielectric acoustic mirror. Using a large number of layers reduces the bandwidth of the mirror and complicates its fabrication.
By using metal and dielectric layers to make an acoustic mirror, the impedance difference can be increased considerably, but so doing introduces a large capacitance provided by the bottom electrode and the top metal layer of the mirror. As illustrated in FIG. 1, such capacitance degrades the performance of filters consisting of two or more resonators on a single substrate; the top metallic layer of the mirror creates a capacitance from each bottom electrode to all the other bottom electrodes in a filter, providing a parasitic capacitive coupling between the resonators.
What is needed is an acoustic-mirror type of resonator in which the acoustic mirror consists of alternating metallic and dielectric layers so as to provide good reflectivity with a relatively small number of layers, but does not introduce capacitive coupling to other resonators formed on the same substrate and using the same acoustic mirror.
Accordingly, the present invention provides a method of fabricating a multi-resonator bulk acoustic wave (BAW) filter and also a filter provided by such a method, the filter having a plurality of layers of materials serving as an acoustic mirror for a plurality of resonator sections, each resonator section including at least a top electrode and a bottom electrode sandwiching a piezolayer, the method including the steps of: choosing dielectric materials for some of the layers of materials serving as the acoustic mirror and metallic materials for the others of the layers; and providing at least one of the metallic layers via a fabrication procedure in which the metallic layer is patterned into distinct portions by an etching process that removes enough of the metallic layer between where different resonator sections are to be placed as to provide electrical isolation between the portions of the layer beneath the different resonator sections; thereby providing a multi-resonator BAW filter with reduced capacitive coupling between resonators, compared to the capacitive coupling present in a multi-resonator BAW filter fashioned in a similar manner except excluding the step of etching to pattern any metallic layers of the similarly fashioned acoustic mirror.
In a further aspect of the invention, all of the metallic layers are patterned into distinct portions so as to provide electrical isolation between the portions of the all of the layers beneath the different resonator sections.